It seems some people get so stuck in their opinions, the most important thing to them is they stand their ground and defend to their death if necessary (ok, maybe not that far, but...), that original opinion and never stop for a second to step back and to to look at all sides and consider reassessing what's happen. "I'm right!! I'm right!! I'm right..!! don't nobody tell me I'm wrong because I'm right!!" Sad.
This is NOT bait-and-switch. For it to be bait-and-switch, there's a requirement of willful and intentional mispresentation by the party providing the goods or services. That's NOT what happened here.
As I (and I believe the majority) see it, GA made an offer of 600 EU with the assumption the OP would be delayed overnight. I am certain the GA did not think "we need volunteers to bump, I'm going to offer 600 EU, but later rescind that and only offer 300 EU". That would be bait-and-switch.
There was much chaos and such getting the plane boarded and on its way, for the GA to be fully aware of all the various bump options is going beyond reasonable expectations. As it turns out, a flight was available to put the bumpees on and incur only a 45-min delay, IMHO, 300 EU is quite a good deal in this case.
If some kind of written agreement had been handed out and signed off on, this would be a different matter. It's why I think things like mistake fares need to be honored-- a<mistake> amount is advertised, money is exchanged, an agreement is made in writing <albeit electronically>. But there's a purely human factor here-- a GA who's difficult job is to get that plane off. Like many here have already pointed out, agreeing to a VDB means giving up some things, including boarding at a time where there's enough overhead luggage space. In the end, it wasn't necessary to stay overnight and put up with the hassles of returning a day later.
In the words of Howie Mandel, the OP got, "a good deal".